This invention relates to photoelectric transducer modules, also known as "photovoltaic" or simply "photocell" modules, and arrays incorporating a plurality of photocells such as solar panels and the like.
A critical consideration when assembling photocell modules, as represented by the solar cell modules and panels described in U.S. Pat. Nos. 3,957,537, 4,224,081, 4,322,261 and 4,331,494, is selection of the pottant material for encapsulating the fragile semi-conductor photocell wafers or disks employed in the modules. The pottant material must surround the semi-conductive disks and their connecting circuitry and support the disks between a transparent, light-transmissive first sheet and a second backing sheet. The backing sheet may also be transparent and light transmissive, but in most photocell assemblies comprises one or more rigid, semi-rigid or flexible support or load-bearing opaque sheets of glass, metal, fibrous material and/or plastic. The completed assembly is generally laminar and sometimes requires the application of primers or adhesives to one or more of the sheets to bind the various components together in a stable manner. Considerable technology has been developed in recent years as photocell assemblies have found increasing uses and achieved popularity in a wide variety of environments, from rooftop solar panels to micro-assemblies such as photocell energized watches and cameras.
The pottant material for encapsulating the photocell wafers must provide electrical insulation of the wafers and connecting circuitry, mechanical protection for the wafer themselves, corrosion protection for the metal contacts, and physical integrity of the circuitry and photocell assembly over an economically useful life of the assembly (usually 20 years in the case of roof-top solar panels). More specifically, the pottant material must be highly transparent and light-transmissive, electrically insulating, must be sufficiently soft to cushion the photocell wafers and to dampen vibration, must protect the cells from stresses due to thermal expansion differences and external impact, must have oxidative, hydrolytic and other chemical stability, and must have good adhesive properties. If the photocell assemblies are used in environments subject to changing conditions of temperature and weather, the pottant must have a glass transition temperature (T.sub.g) below the lowest temperature extreme which the module might experience, and must also exhibit no significant mechanical creep at upper operating temperature extremes so that the laminar assembly will remain intact. Accordingly, for most rooftop solar cell applications and similar applications subject to the elements in northerly regions of the western hemisphere, the glass transition temperature must be below about -40.degree. C. and no significant mechanical creep should be experienced up to about 90.degree. C.
Additional considerations in selecting an effective photocell pottant include melt viscosity/temperature relationship, the speed and ease with which photocell assemblies may be produced, and capability of recycling scrap pottant material. Concerning melt viscosity/temperature relationship, photocell assemblies are commonly produced by vacuum lamination in which the steeper the melt viscosity/ temperature curve for the pottant the greater the ease of processing. The pottant material, usually in the form of a polymeric sheet, must be dry and non-tacky during the initial evacuation step so as not to entrap air between the layers. However, the pottant must melt during the lamination to provide sufficient fluidity for penetration and wetting of all the irregularities of the cell circuit.
The foregoing and other considerations in selecting an effective pottant material for photocell assemblies are described in the technical literature such as the paper "Encapsulant Material Requirements for Photovoltaic Modules" by Katherine J. Lewis, American Chemical Society Symposium Series, Polymers for Solar Energy, 220, 367 (1983).
The major materials presently employed as photocell pottants are cross-linked ethylene-vinyl acetate (EVA) copolymer, polyvinyl butyral (PVB) and epoxy resin. All of these materials are deficient as photocell pottants in various ways. EVA copolymers have very low melt temperatures and therefore require cross-linking for improvement of their melt viscosity/temperature relationships, particularly for reducing photocell assembly production time. Also, EVA pottants contain residual peroxides and volatile by-products which lead to out-gassing during photocell assembly production and therefore cause voids in the pottant and resultant de-lamination. The presence of volatile hydrolysis by-products in these pottants also contributes to product instability. Moreover, EVA pottants inherently are not sufficiently adhesive and require a primer for adequate bonding to other layers in the photocell assembly. Lastly, EVA pottants have crystalline regions which scatter light, thereby reducing optical clarity, and since the pottants are chemically cross-linked, scrap material cannot be recycled for subsequent photocell assembly production.
PVB pottants suffer from poor chemical resistance and hydrolytic stability, a tendency to absorb moisture, and inferior electrical properties. In addition, PVB requires a plasticizer for processability, but plasticizers reduce the volume resistivity of a polymer, leading to reduced electrical insulating properties. This can be somewhat offset by the use of a high volume resistivity layer in the photocell assembly, such as a polyethylene terephthalate film, but this complicates photocell assembly production and adds to cost.
Epoxy resin pottants have the disadvantages of chemical cross-linking, relatively complicated and long photocell assembly production times, high tensile modulus, and are incapable of being recycled for subsequent use in photocell assembly production.
Becaus of the deficiencies of the foregoing pottant materials, attention has focused on thermoplastic elastomers. Generally, these materials have steep melt viscosity/temperature curves for good processability, have high cohesive strength and toughness, and would be expected to lead to reduced photocell assembly times since peroxide or other types of chemical cross-linking are not required. However, these advantages in practice tend to be cancelled or offset by generally poor oxidative stability and reduced optical clarity, as evident in conventional styrene-diene thermoplastic elastomers.